In mass data storage devices, data stored on a magnetic or optical medium are read by a head that moves relative to the medium. The head generates an analog signal based on the stored data. Because of the density of the data stored on the medium, the value of the read signal at any point in time is the product of a number of stored data values, not just the data value associated with the current time point. Thus, the current value of the analog signal is a function of data values read from the medium before, at and even after the current time point. This function is known as the channel response of the data storage device.
In the prior art, the read signal was conditioned by an equalizer so that the equalized read signal fits a desired function, known as a target. Such functions are typically described using the delay operator D where D is raised to a power to indicate the time point of a data value. Thus, D indicates the preceding data value, D2 indicates the second preceding value, D3 indicates the third preceding value, and so forth. Examples of prior art targets include the PR4 target defined as (1−D)(1+D), the EPR4 target defined as (1−D)(1+2D+D2) and the E2PR4 target defined as (1−D)(1+3D+3D2+D3).
In the past, equalization targets have generally been selected based on their spectral properties and their correspondence to the natural channel response of the storage device. Although these factors provide some criteria for selecting from the unlimited number of possible targets, they do not ensure that the target will provide the best results. Thus, a better method is needed for identifying an equalization target.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.